In reinforced concrete the steel reinforcement is normally protected against corrosion by the alkaline environment in the concrete mass. Gradually however the alkalinity is reduced by the action of carbon dioxide and other gases in the atmosphere such as oxides of sulphur. The term given to this process is carbonation and concrete which has been subjected to the action of atmospheric gases is referred to as carbonated.
The effect is a gradual decrease in the pH and, if the process is allowed to continue, the pH will continue to fall and when it reaches a value of about 9.5 the steel is no longer protected against corrosion. Corrosion of the reinforcement may then take place leading to weakening of the reinforcement and spalling of the concrete.
A process for increasing the alkalinity of concrete which has been carbonated has been previously described, for example in European Patent No. 264,421 and U.S. Pat. No. 4,865,702. In this process an electric current is passed between an external electrode which is in contact with an alkaline electrolyte applied to the external surface of the concrete and the internal reinforcement of the concrete as cathode.
During the process there are two effects: the alkaline electrolyte moves into the concrete and the alkalinity is increased in the region of the cathode. The benefit of this is that the steel is repassivated and a layer of electrolyte which has permeated into the concrete maintains a cover zone over and around the steel of sufficiently high pH to keep the steel passive. This process has become known as realkalization.
As the alkaline electrolyte for this process there is proposed in European Patent No. 264,421 an aqueous solution of calcium, sodium and/or potassium salts either in a liquid or absorbed in a porous medium such as rock wool, cellulose, sawdust, sand, clay and the like or the electrolyte can be strongly retarded concrete, mortar, cement grout or lime paste. GB Patent Aplication No. 2,271,123A discloses a process in which sodium carbonate or sodium borate is used in the aqueous electrolyte. However in the operation of the process on a commercial scale the alkaline electrolyte that has been used is an aqueous solution of sodium carbonate.